Recursive Functions
A recursive function solves a problem by calling itself on a smaller version, like opening Russian dolls until you reach the tiniest one. It needs a stopping point so it does not go on forever.
In this page:
Syntax
def function_name(n: Int): Int = {
if (n <= 1) base_value // base case
else n * function_name(n - 1) // recursive step
}Recursive Functions
A Scala function can call itself, producing recursion, as long as there's a base case that stops it. Recursive functions need an explicit return type annotation (unlike simple functions where it can often be inferred), since the compiler can't infer a recursive function's return type from its own body.
The @tailrec annotation can be added to verify, at compile time, that a recursive call is in tail position and will be optimized into a loop instead of growing the call stack.
Warning:
A recursive function's return type must be explicitly declared -- Scala cannot infer it from a body that calls the function itself.
Example: Recursive Functions
object Main extends App {
def factorial(n: Int): Int = {
if (n <= 1) 1
else n * factorial(n - 1)
}
println(factorial(5))
}Login to try C/C++/Java/PHP code in the editor
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Leaving out the base case, so
factorialcalls itself forever and ends in aStackOverflowError. - Using
Intforfactorial(20), which overflows silently and gives a wrong number; useLongorBigInt. - Forgetting that recursion needs a declared return type, so
def factorial(n: Int) = ...gives an error.
Chapter Summary
- Functions have parameters and return types, and can use default parameters, named arguments, and varargs.
- Nested functions keep helpers local.
- Recursive functions call themselves to solve problems.
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